Modified pe separator with low decomposition voltage and high lithium supplement capacity and preparation method thereof

Abstract

The application relates to the technical field of lithium battery diaphragm, in particular to a modified PE diaphragm with low decomposition voltage and high lithium supplement capacity and a preparation method thereof. The preparation method comprises the following steps: mixing C3N4, a molybdenum source, a cobalt source and citric acid to obtain a precursor; sintering the precursor under a protective atmosphere to obtain a composite catalyst; mixing the composite catalyst, a conductive agent, a binder, lithium oxalate and a solvent to obtain a slurry; and coating the slurry on the surface of a PE diaphragm to obtain the modified PE diaphragm with low decomposition voltage and high lithium supplement capacity after drying. The composite catalyst is obtained by sintering a multiphase precursor, the slurry is mixed with a conductive agent, a binder, lithium oxalate and the like, and is coated on the PE diaphragm, so that the decomposition potential of lithium oxalate can be greatly reduced, the problem that lithium oxalate is difficult to supplement lithium efficiently in a ternary positive electrode system can be effectively solved, the specific capacity of lithium oxalate released at low voltage can be improved, and the effect of high lithium supplement capacity is achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium battery separator, in particular to a modified PE separator with low decomposition voltage and high lithium supplement capacity and a preparation method thereof. BACKGROUND

[0002] With the rise of electric vehicles and the rapid development of portable electronic devices, traditional battery technology has been difficult to meet the increasing performance requirements. In this context, lithium-ion batteries are widely used in various electronic devices due to their cleanliness, safety and higher energy density than traditional batteries. However, during the first charge and discharge process of lithium batteries, a layer of SEI film (solid-state electrolyte interface film) will inevitably be formed on the surface of the negative electrode material. This film formation process irreversibly consumes lithium ions, directly leading to a decrease in the first cycle coulombic efficiency of the battery, and also causing the actual overall reversible capacity (i.e. energy density) of the battery to decrease.

[0003] To overcome this problem, pre-lithiation technology has emerged. This technology introduces an additional lithium source before the first charge and discharge of the battery to pre-compensate for the lithium ions consumed by the subsequent SEI film formation, thereby effectively improving the first cycle coulombic efficiency and final energy density of the battery, and also helping to improve the cycle performance. Currently, common pre-lithiation methods mainly include metal lithium sheet pre-lithiation, chemical pre-lithiation and electrochemical pre-lithiation, etc. Among them, chemical pre-lithiation has received more attention due to its relative ease of integration into existing production processes. This method usually uses a lithium-containing reagent with strong reducing properties (such as n-butyllithium) to directly deposit active lithium onto the surface of the negative electrode material through an oxidation-reduction reaction. However, such strong reducing agents (such as n-butyllithium) have very high chemical activity, are extremely sensitive to air and moisture, and are flammable and explosive, posing a serious safety hazard, which limits their practical application.

[0004] In contrast, lithium oxalate (Li2C2O4) is relatively stable in air and is not prone to severe oxidation or hydrolysis reactions, and is considered a safer type of chemical pre-lithiation candidate material with more practical prospects. However, lithium oxalate has the problem of a too high decomposition voltage (usually above 4.7V vs. Li + / Li), which hinders its further application. To solve this problem, researchers have tried to introduce catalysts to reduce its decomposition voltage, but the decomposition voltage of lithium oxalate has not yet been reduced to a level that can be directly applied to lithium batteries with high-efficiency lithium supplement for ternary positive electrode materials. SUMMARY

[0005] The present application aims to solve the problem of insufficient reduction of lithium oxalate decomposition voltage in the prior art, and provides a modified PE separator with low decomposition voltage and high lithium supplement capacity and a preparation method thereof, wherein a composite catalyst is obtained by sintering a multi-phase precursor, a slurry of a conductive agent, a binder, lithium oxalate and the like is mixed, and then coated on a PE separator, so as to greatly reduce the decomposition potential of lithium oxalate and effectively solve the problem of difficult efficient lithium supplement of lithium oxalate in a ternary positive electrode system.

[0006] To achieve the above-mentioned purpose, the present application provides a preparation method of a modified PE separator with low decomposition voltage and high lithium supplement capacity, comprising the following steps:

[0007] S1. mixing C3N4, a molybdenum source, a cobalt source and citric acid to obtain a precursor;

[0008] S2. sintering the precursor in a protective atmosphere to obtain a composite catalyst;

[0009] S3. mixing the composite catalyst, a conductive agent, a binder, lithium oxalate and a solvent to obtain a slurry;

[0010] S4. coating the slurry on the surface of the PE separator, and drying to obtain a modified PE separator with low decomposition voltage and high lithium supplement capacity.

[0011] Preferably, the molybdenum source in S1 includes one or more of ammonium molybdate and molybdenum oxide, and the cobalt source includes one or more of cobalt nitrate, cobalt chloride and cobalt acetate.

[0012] Preferably, in S1, the ratio of the amount of substance of carbon in C3N4 to the amount of substance of molybdenum in the molybdenum source is (10-12):(1-1.2); the ratio of the amount of substance of molybdenum in the molybdenum source to the amount of substance of cobalt in the cobalt source is 10:(1-7); and the total amount of substance of molybdenum in the molybdenum source and cobalt in the cobalt source to the amount of substance of citric acid is (0.5-1.5):(1-2).

[0013] Preferably, in S1, the mixing is performed in a mixed solution of ethanol and water, and the volume ratio of ethanol to water is (0.5-1.5):(0.5-1.5).

[0014] Preferably, the protective atmosphere in S2 includes one or more of argon and hydrogen; the heating rate of sintering is 8℃ / min-12℃ / min, the target temperature of sintering is 780℃-820℃, and the holding time of sintering is 1.5h-2.5h.

[0015] Preferably, the conductive agent in S3 includes one or more of carbon black conductive agent and graphene composite conductive agent; the binder includes polyvinylidene fluoride; and the solvent includes one or more of N-methylpyrrolidone and dimethylformamide.

[0016] Preferably, the mass ratio of the composite catalyst, the conductive agent, the binder and the lithium oxalate in S3 is (15-25):(5-15):(2-8):(60-70); and the mass fraction of the solid substances in the slurry is 15%-25%.

[0017] Preferably, in S4, the ratio of the thickness of the coating to the thickness of the PE separator is (1-2):(10-20).

[0018] Preferably, in S4, the drying comprises sequentially performing first drying and second drying.

[0019] The temperature of the first drying is 20-30 DEG C, and the time of the first drying is 2-4 hours; the temperature of the second drying is 50-70 DEG C, and the time of the second drying is 11-13 hours.

[0020] The application further provides a modified PE separator with low decomposition voltage and high lithium supplement capacity, which is prepared by the preparation method of the modified PE separator with low decomposition voltage and high lithium supplement capacity.

[0021] The application has the following beneficial effects:

[0022] 1. The application provides a preparation method of a modified PE separator with low decomposition voltage and high lithium supplement capacity, which comprises the following steps: obtaining a composite catalyst by sintering a multiphase precursor, mixing a conductive agent, a binder, lithium oxalate and the like to form a slurry, and coating the slurry on a PE separator, so that the decomposition potential of lithium oxalate can be greatly reduced, the problem that lithium oxalate is difficult to supplement lithium in a ternary positive electrode system can be effectively solved, the specific capacity of lithium oxalate released at a low voltage can be improved, and the effect of high lithium supplement capacity can be achieved.

[0023] 2. In the composite catalyst, the empty orbit of molybdenum carbide can adsorb and catalyze the lone pair electrons of lithium oxalate, the carbon structure with high conductivity can establish a fast transmission channel for the migration of electrons and ions, the introduced cobalt element can change the electron cloud distribution and surface chemical environment of molybdenum carbide, increase the active center, reduce the energy barrier of lithium release of lithium oxalate, accelerate the reaction kinetics between lithium ions and active substances, and promote the embedding and de-embedding process of lithium ions, so that the lithium supplement efficiency can be improved, the battery can complete the charging and discharging process more quickly, and the rate performance of the battery can be improved.

[0024] Meanwhile, the cobalt element can also fill the lattice defects or gap positions of molybdenum carbide, support and reinforce the lattice structure, enhance the stability of the crystal structure of molybdenum carbide, and further improve the overall stability of the coating. In addition, the presence of the lithium supplement coating (the coating formed after the slurry is dried) can improve the mechanical strength of the PE separator, and in combination with the promotion of the reaction kinetics by the composite catalyst, the assembled battery has excellent cycle capacity retention rate and high specific capacity, and excellent cycle stability.

[0025] 3. The lithium oxalate, catalyst, etc. are loaded on the PE separator in a surface coating manner, this lithium supplementing manner has high flexibility, the loading amount and distribution can be adjusted according to the actual demand of the battery, and the lithium supplementing function can be realized without significantly increasing the volume and weight of the battery, which is beneficial to the realization of high energy density of the lithium battery. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 is a TEM characterization graph of the composite catalyst in Example 1 of the present application;

[0027] Figure 2 is an EDS mapping graph of the composite catalyst in Example 1 of the present application;

[0028] Figure 3 is a local enlarged graph of the TEM characterization of the composite catalyst in Example 1 of the present application;

[0029] Figure 4 is a SEM characterization graph of the composite catalyst in Example 1 of the present application;

[0030] Figure 5 is an XRD characterization graph of the composite catalyst in Example 1 of the present application;

[0031] Figure 6 is an XRD characterization comparison graph of the composite catalyst in Example 1-Example 2, Comparative Example 1 of the present application;

[0032] Figure 7 is an XPS characterization graph of the composite catalyst in Comparative Example 1 of the present application, Figure 7 a in is a C 1s spectrum of the composite catalyst in Comparative Example 1, Figure 7 b in is a Mo 3d spectrum of the composite catalyst in Comparative Example 1, Figure 7 c in is a Co 2p spectrum of the composite catalyst in Comparative Example 1;

[0033] Figure 8 is an XPS characterization graph of the composite catalyst in Example 1 of the present application, Figure 8 a in is a C 1s spectrum of the composite catalyst in Example 1, Figure 8 b in is a Mo 3d spectrum of the composite catalyst in Example 1, Figure 8 c in is a Co 2p spectrum of the composite catalyst in Example 1;

[0034] Figure 9 is a contact angle graph of the modified PE separator with low decomposition voltage and high lithium supplementing capacity in Example 1 of the present application;

[0035] Figure 10 is a contact angle graph of the modified PE separator with low decomposition voltage and high lithium supplementing capacity in Example 2 of the present application;

[0036] Figure 11 is the contact angle graph of the modified PE separator in the present application comparative example 1;

[0037] Figure 12 is the stress-strain curve comparison graph of different modified PE separators in the present application;

[0038] Figure 13 is the critical current density comparison graph of different modified PE separators in the present application;

[0039] Figure 14 is the lithium ion transference number change graph of the modified PE separator with low decomposition voltage and high lithium supplement capacity in the present application example 1; Figure 14 the large graph in is the i-t curve graph corresponding to example 1, Figure 14 the small graph in is the impedance graph before and after polarization corresponding to example 1;

[0040] Figure 15 is the lithium ion transference number change graph of the modified PE separator with low decomposition voltage and high lithium supplement capacity in the present application example 2; Figure 15 the large graph in is the i-t curve graph corresponding to example 2, Figure 15 the small graph in is the impedance graph before and after polarization corresponding to example 2;

[0041] Figure 16 is the lithium ion transference number change graph of the modified PE separator in the present application comparative example 1; Figure 16 the large graph in is the i-t curve graph corresponding to comparative example 1, Figure 16 the small graph in is the impedance graph before and after polarization corresponding to comparative example 1;

[0042] Figure 17 is the lithium oxalate decomposition voltage platform curve comparison graph in the activation process of different button batteries in the present application;

[0043] Figure 18 is the dQ / dV curve comparison graph in the activation process of different button batteries in the present application;

[0044] Figure 19 is the charge-discharge curve comparison graph in the activation process of different button batteries in the present application;

[0045] Figure 20 is the first circle charge lithium supplement capacity comparison graph in the activation process of different button batteries in the present application;

[0046] Figure 21 is the cycle stability comparison graph of different lithium symmetric batteries in the present application;

[0047] Figure 22 is the cycle curve comparison graph of different full batteries in the present application. DETAILED DESCRIPTION

[0048] The application provides a preparation method of a modified PE diaphragm with low decomposition voltage and high lithium supplement capacity, comprising the following steps:

[0049] S1. mixing C3N4, a molybdenum source, a cobalt source and citric acid to obtain a precursor;

[0050] S2. sintering the precursor under a protective atmosphere to obtain a composite catalyst;

[0051] S3. mixing the composite catalyst, a conductive agent, a binder, lithium oxalate and a solvent to obtain a slurry;

[0052] S4. coating the slurry on the surface of the PE diaphragm, and drying to obtain the modified PE diaphragm with low decomposition voltage and high lithium supplement capacity.

[0053] In the application, the preparation method of C3N4 in S1 comprises the following steps: sufficiently grinding melamine and sintering to obtain C3N4.

[0054] In the application, the sintering temperature increasing rate is 2-3 ℃ / min, the target sintering temperature is 500-600 ℃, and the sintering holding time is 5-7 h.

[0055] In the application, the molybdenum source in S1 comprises one or more of ammonium molybdate and molybdenum oxide, and the cobalt source comprises one or more of cobalt nitrate, cobalt chloride and cobalt acetate.

[0056] In the application, the ammonium molybdate comprises ammonium molybdate tetrahydrate, and the cobalt nitrate comprises cobalt nitrate hexahydrate.

[0057] In the application, in S1, the ratio of the amount of substance of carbon in C3N4 to the amount of substance of molybdenum in the molybdenum source is (10-12):(1-1.2); the ratio of the amount of substance of molybdenum in the molybdenum source to the amount of substance of cobalt in the cobalt source is 10:(1-7); and the ratio of the total amount of substance of molybdenum in the molybdenum source and cobalt in the cobalt source to the amount of substance of citric acid is (0.5-1.5):(1-2).

[0058] In the application, the ratio of the amount of substance of molybdenum in the molybdenum source to the amount of substance of cobalt in the cobalt source is 10:(1-7), preferably 10:3. The cobalt source is used to assist the molybdenum source in electron transfer; when the cobalt content is too low, the ability of assisting electron transfer is weakened, and thus the catalytic effect is reduced; when the cobalt content is too high, the relative content of molybdenum carbide in a unit volume is reduced when the total amount of the composite catalyst is constant, and thus the catalytic effect is reduced.

[0059] In the application, in S1, the mixing is carried out in a mixed solution of ethanol and water, the volume ratio of ethanol to water is (0.5-1.5):(0.5-1.5), and the ratio of the total mass of the mixed solution of ethanol and water to the mass of C3N4 is (15-20):1.

[0060] In the application, in S1, the temperature of mixing is 50-70℃, the stirring speed is 500-700rpm, and the time is 1-3h.

[0061] In the application, in S1, after the mixing is completed, drying and ball milling are sequentially carried out to obtain a precursor.

[0062] In the application, the drying temperature is 110-130℃, the drying time is 11-13h, the ball-to-material ratio of the ball milling is (3-5):1, the ball milling speed is 300-500rpm, and the ball milling time is 2-4h.

[0063] In the application, the protective atmosphere in S2 includes one or more of argon and hydrogen, the heating rate of sintering is 8-12℃ / min, the target temperature of sintering is 780-820℃, and the holding time of sintering is 1.5-2.5h.

[0064] In the application, the conductive agent in S3 includes one or more of carbon black conductive agent and graphene composite conductive agent, the binder includes polyvinylidene fluoride, and the solvent includes one or more of N-methylpyrrolidone and dimethylformamide.

[0065] In the application, the mass ratio of the composite catalyst, the conductive agent, the binder, and lithium oxalate in S3 is (15-25):(5-15):(2-8):(60-70), and the mass fraction of solid substances in the slurry is 15-25%.

[0066] In the application, the mixing in S3 includes the following steps: first mixing the composite catalyst, the conductive agent, the binder, and lithium oxalate, then adding the solvent, and then ball milling to complete the mixing.

[0067] In the application, the ball-to-material ratio of the ball milling is (2-4):1, the ball milling speed is 300-500rpm, and the ball milling time is 5-7h.

[0068] In the application, the coating method in S4 includes single-sided coating by using a wire bar coating method, and the wire bar type of the wire bar coating method includes OSP-1.5.

[0069] In the application, in S4, the ratio of the thickness of the coating to the thickness of the PE separator is (1-2):(10-20).

[0070] In the present application, in S4, the drying comprises sequentially performed first drying and second drying.

[0071] In the present application, the temperature of the first drying is 20-30℃, the time of the first drying is 2-4h; the second drying is performed under vacuum condition, the temperature of the second drying is 50-70℃, the time of the second drying is 11-13h.

[0072] The present application also provides a modified PE separator with low decomposition voltage and high lithium supplement capacity, which is prepared by the preparation method of the modified PE separator with low decomposition voltage and high lithium supplement capacity.

[0073] The present application is further described below in conjunction with the accompanying drawings and examples. Unless otherwise defined, the technical terms or scientific terms used in the present application shall be understood as the usual meanings understood by those skilled in the art. The features mentioned in the present application or the features mentioned in the specific examples can be combined arbitrarily, and these specific examples are only used to illustrate the present application and are not used to limit the scope of the present application.

[0074] The raw materials used in the embodiments of the present application are commercially available, wherein: the conductive agent includes acetylene black, ketjen black, kena D5315, and lry LA205, the kena D5315 is purchased from Xiamen Kena Graphene Technology Co., Ltd., and the lry LA205 is purchased from Sichuan Lry New Material Technology Co., Ltd.

[0075] The PE separator is purchased from Dongguan Keluode New Energy Technology Co., Ltd., and the thickness is 12 μm.

[0076] Example 1

[0077] The present embodiment provides a preparation method of a modified PE separator with low decomposition voltage and high lithium supplement capacity, comprising the following steps:

[0078] After the melamine is fully ground, sintering is performed under air atmosphere, the heating rate of sintering is set to 2.5℃ / min, the target temperature is 550℃, and the holding time for reaching the target temperature is 6h, to obtain C3N4.

[0079] C3N4, ammonium molybdate tetrahydrate, cobalt nitrate hexahydrate, citric acid, ethanol and water were mixed at 60℃ with stirring at 600rpm for 2h, wherein the ratio of the amount of substance of carbon in C3N4 to the amount of substance of molybdenum in the molybdenum source was 11:1.1; the ratio of the amount of substance of molybdenum in the molybdenum source to the amount of substance of cobalt in the cobalt source was 10:3; the ratio of the total amount of substance of molybdenum in the molybdenum source and cobalt in the cobalt source to the amount of substance of citric acid was 1:1.5, the volume ratio of ethanol to water was 1:1, the ratio of the total mass of the mixed solution of ethanol and water to the mass of C3N4 was 17.5:1, after mixing, the mixture was dried at 120℃ for 12h, and finally the dried powder was ball milled, the ball-to-material ratio was set to 4:1, the rotation speed was 400rpm, and the time was 3h to obtain the precursor.

[0080] The precursor was heated to 800℃ at a heating rate of 10℃ / min under an argon atmosphere, and sintered, the holding time at 800℃ was set to 2h to obtain the composite catalyst.

[0081] The composite catalyst, acetylene black, ketjen black, kynol D5315, leil LA205, polyvinylidene fluoride and lithium oxalate were mixed (the mass ratio of the composite catalyst, acetylene black, ketjen black, kynol D5315, leil LA205, polyvinylidene fluoride and lithium oxalate was 20.36:6:2:0.2:0.2:5.08:66.16), N-methyl pyrrolidone was added, and ball milling was performed, the ball-to-material ratio was set to 3:1, the rotation speed was 400rpm, and the time was 6h to obtain the slurry (the mass fraction of solid substances in the slurry was 20%).

[0082] The slurry was coated on the surface of the PE separator (single-sided coating was performed by wire bar coating, the wire bar type was OSP-1.5), the coating thickness was 1.5μm, and after coating, drying was performed, first at 25℃ for 3h, and then at 60℃ under vacuum for 12h to obtain the modified PE separator with low decomposition voltage and high lithium supplement capacity.

[0083] Example 2

[0084] The present example provides a preparation method of a modified PE separator with low decomposition voltage and high lithium supplement capacity, which is basically the same as example 1, except that the ratio of the amount of substance of molybdenum in the molybdenum source to the amount of substance of cobalt in the cobalt source was modified to 10:6.

[0085] Comparative Example 1

[0086] The present comparative example provides a preparation method of a modified PE separator, which comprises the following steps:

[0087] After the melamine is ground sufficiently, sintering is performed under an air atmosphere, the heating rate of sintering is set to be 2.5℃ / min, the target temperature is 550℃, and the holding time for reaching the target temperature is 6h, to obtain C3N4.

[0088] The C3N4 and ammonium molybdate tetrahydrate are mixed, the ratio of the amount of substance of carbon in the C3N4 to the amount of substance of molybdenum in the molybdenum source is 11:1.1, the mixed powder is ball milled, the ball-to-material ratio of ball milling is set to be 4:1, the rotating speed is 400rpm, and the time is 3h, to obtain a precursor.

[0089] The precursor is heated to 800℃ at a heating rate of 10℃ / min under an argon atmosphere, sintering is performed, the holding time at 800℃ is set to be 2h, to obtain a composite catalyst.

[0090] The composite catalyst, acetylene black, ketjen black, kane D5315, leil LA205, polyvinylidene fluoride and lithium oxalate are mixed (the mass ratio of the composite catalyst, acetylene black, ketjen black, kane D5315, leil LA205, polyvinylidene fluoride and lithium oxalate is 20.36:6:2:0.2:0.2:5.08:66.16), N-methyl pyrrolidone is added, ball milling is performed, the ball-to-material ratio of ball milling is set to be 3:1, the rotating speed is 400rpm, and the time is 6h, to obtain a slurry (the mass fraction of solid substances in the slurry is 20%).

[0091] The slurry is coated on the surface of a PE separator (single-sided coating is performed by using a wire bar coating method, the wire bar type is OSP-1.5), the coating thickness is 1.5μm, and drying is performed after coating, first at 25℃ for 3h, and then at 60℃ under vacuum for 12h, to obtain a modified PE separator.

[0092] Experimental Example 1

[0093] The composite catalyst prepared in Example 1 is characterized by transmission electron microscopy (TEM), energy dispersive spectrometer (EDS) and scanning electron microscopy (SEM). The TEM characterization graph of the composite catalyst in Example 1 is shown in Figure 1 , the EDS mapping graph of the composite catalyst in Example 1 is shown in Figure 2 , the local magnified TEM characterization graph of the composite catalyst in Example 1 is shown in Figure 3 , and the SEM characterization graph of the composite catalyst in Example 1 is shown in Figure 4 . As can be seen from Figures 1-3 , Mo2C and Co are in close contact, and Mo2C and Co particles are distributed between phases, which is conducive to electron transfer. As can be seen from Figure 4 , the composite catalyst powder is in a spherical morphology, which is more conducive to contact with lithium oxalate for catalysis.

[0094] Experiment Example 2

[0095] The composite catalysts prepared in Examples 1-2 and Comparative Example 1 were characterized by X-ray diffraction (XRD). The XRD pattern of the composite catalyst in Example 1 is shown below. Figure 5 As shown; XRD characterization comparison diagrams of the composite catalysts in Examples 1-2 and Comparative Example 1, as shown. Figure 6 As shown. From Figures 5-6 As can be seen from the data, the obvious crystal structure of the composite catalyst in Example 1 is Mo2C, Co, and MoC. Comparing Example 1 and Example 2 with Comparative Example 1, it was found that after introducing Co, the crystallinity of MoC decreased, while the crystallinity of Mo2C and Co increased.

[0096] Experimental Example 3

[0097] The composite catalysts prepared in Example 1 and Comparative Example 1 were characterized by X-ray photoelectron spectroscopy (XPS). The XPS characterization patterns of the composite catalysts in Example 1 and Comparative Example 1 are shown below. Figure 7 As shown; Figure 7 In the figure, 'a' represents the C1s spectrum of the composite catalyst in Comparative Example 1. Figure 7 In the diagram, b represents the Mo 3d spectrum of the composite catalyst in Comparative Example 1. Figure 7 In the figure, c represents the Co 2p spectrum of the composite catalyst in Comparative Example 1.

[0098] XPS characterization of the composite catalyst in Example 1, as shown below. Figure 8 As shown; Figure 8 In this diagram, 'a' represents the C1s spectrum of the composite catalyst in Example 1. Figure 8 In the image, b represents the Mo 3d spectrum of the composite catalyst in Example 1. Figure 8 In the figure, c represents the Co 2p spectrum of the composite catalyst in Example 1.

[0099] from Figures 7-8 From the results, we can see that the C 1s spectrum shows that, compared to Comparative Example 1, the peak intensity of C-Mo decreased after the introduction of Co in Example 1, indicating that the introduction of Co affected the bonding environment between C and Mo. The Mo 3d spectrum shows that... 2+ and Mo 4+ The peak is a characteristic peak of Mo-C, Mo 6+ The peak is a result of partial surface oxidation. Mo 2+ Mo 4+ 3D 5 / 2 The orbital (main peak on the right) binding energies underwent negative shifts of 0.1 eV and 0.53 eV, respectively, indicating an increase in the electron density around Mo atoms in Mo2C.

[0100] The mechanism is that when the electron density increases, the shielding effect between electrons is enhanced, which weakens the attraction of the atomic nucleus to the outer electrons, resulting in a decrease in the binding energy and a right shift of the peak. Co 2p spectrum: after the introduction of Co in Example 1, the Co element exhibits the state of Co 0 , Co 2+ , Co 3+ , and Co 0 represents elemental Co. The appearance of Co 2+ , Co 3+ indicates that the form of Co is not only elemental, but also a small amount of Co atoms enter the Mo2C lattice and interact. Co is reduced from cobalt nitrate hexahydrate and tends to be metallic, and the electron density around it decreases. The opposite displacement of the binding energy between Co and Mo confirms the electron transfer in the Co-Mo2C system.

[0101] Experimental Example 4

[0102] The modified PE separator prepared in Example 1-Example 2 with low decomposition voltage and high lithium supplement capacity, the modified PE separator prepared in Comparative Example 1 were respectively subjected to electrolyte contact angle test (the electrolyte was (KLD-LP06): 1.0 mol / L lithium hexafluorophosphate (LiPF6), the solvent was ethylene carbonate (EC) and dimethyl carbonate (DMC) mixed in a volume ratio of 1:1, and 5% fluoroethylene carbonate (FEC) was added).

[0103] The contact angle diagram of the modified PE separator with low decomposition voltage and high lithium supplement capacity in Example 1 is shown in FIG. 1; the contact angle diagram of the modified PE separator with low decomposition voltage and high lithium supplement capacity in Example 2 is shown in FIG. 2; and the contact angle diagram of the modified PE separator in Comparative Example 1 is shown in FIG. 3. As can be seen from FIGS. 1-3, the modified PE separator in Example 1 has a smaller electrolyte contact angle and better wettability to the electrolyte. Figure 9 Figure 10 Figure 11 Figures 9-11 As can be seen from FIGS. 1-3, the modified PE separator in Example 1 has a smaller electrolyte contact angle and better wettability to the electrolyte.

[0104] Experimental Example 5

[0105] The modified PE separators prepared in Example 1-Example 2 with low decomposition voltage and high lithium supplement capacity, the modified PE separator prepared in Comparative Example 1 were respectively subjected to stress-strain test. Taking Example 1 as an example, a separator with a width of 2 cm and a length of 10 cm was taken for testing, and the strain rate was set to 10 mm / min. Finally, the stress-strain curve comparison diagram of different modified PE separators is shown in FIG. 4. As can be seen from FIG. 4, compared with Comparative Example 1, the mechanical properties of the modified PE separators in Examples 1 and 2, especially in Example 1, are greatly improved. Figure 12 Figure 12 As can be seen from FIG. 4, compared with Comparative Example 1, the mechanical properties of the modified PE separators in Examples 1 and 2, especially in Example 1, are greatly improved.

[0106] Experimental Example 6​​​​

[0107] The modified PE separators with low decomposition voltage and high lithium supplement capacity prepared in Example 1-Example 2 and the modified PE separator prepared in Comparative Example 1 were respectively assembled into lithium symmetric cells for critical current density (CCD) test. In the test, the current density was increased from 0.01 mAh / cm 2 to 20 mAh / cm 2 at a step, and the change of voltage with current density was recorded. The comparison chart of critical current density of different modified PE separators is shown in Figure 13 From Figure 13 it can be seen that the modified PE separator in Example 1 has higher critical current density and lower overpotential. At the same time, the comprehensive performance of the modified PE separator in Example 1 is the best in the comprehensive performance test of critical current density, overpotential and mechanical property.

[0108] Experimental Example 7

[0109] The modified PE separators with low decomposition voltage and high lithium supplement capacity prepared in Example 1-Example 2 and the modified PE separator prepared in Comparative Example 1 were respectively subjected to lithium ion migration number test: each separator was assembled into a lithium symmetric cell, the impedance value before polarization was measured, then a direct current voltage of 10 mV was applied for constant voltage polarization, and the i-t curve was recorded for 10000 s; after the polarization was completed, the impedance value after polarization was measured again (impedance after polarization).

[0110] The change chart of lithium ion migration number of the modified PE separator with low decomposition voltage and high lithium supplement capacity in Example 1 is shown in Figure 14 The large graph in Figure 14 is the i-t curve graph corresponding to Example 1, Figure 14 and the small graph is the impedance graph before and after polarization corresponding to Example 1.

[0111] The change chart of lithium ion migration number of the modified PE separator with low decomposition voltage and high lithium supplement capacity in Example 2 is shown in Figure 15 The large graph in Figure 15 is the i-t curve graph corresponding to Example 2, Figure 15 and the small graph is the impedance graph before and after polarization corresponding to Example 2.

[0112] The change chart of lithium ion migration number of the modified PE separator in Comparative Example 1 is shown in Figure 16 The large graph in Figure 16 is the i-t curve graph corresponding to Comparative Example 1, Figure 16 and the small graph is the impedance graph before and after polarization corresponding to Comparative Example 1.

[0113] Comparison Figures 14-16It can be seen that the lithium-ion transference number of the modified PE membrane in Example 1 is 0.52, and the lithium-ion transference number of the modified PE membrane in Example 2 is 0.35, which is higher than the lithium-ion transference number of the modified PE membrane 2 in Comparative Example 1 (0.22).

[0114] Experimental Example 8

[0115] The modified PE separators with low decomposition voltage and high lithium replenishment capacity prepared in Examples 1-2, and the modified PE separator prepared in Comparative Example 1, were respectively assembled into button cells with aluminum foil as the positive electrode and lithium sheet as the negative electrode (the separator coating side faces the positive electrode). These button cells were activated at a rate of 0.05C with a charge-discharge voltage range of 2.8V-4.3V.

[0116] A comparison of lithium oxalate decomposition voltage plateau curves during the activation process of different button batteries was obtained, as shown in the figure. Figure 17 As shown; will Figure 17 Differential processing was performed to obtain a comparison graph of dQ / dV curves during the activation process of different button cells, as shown below. Figure 18 As shown. By Figures 17-18 It can be seen that the battery assembled in Example 1 exhibits the lowest lithium oxalate decomposition voltage plateau, with a value of approximately 4.15V. This indicates that the composite catalyst used in Example 1 has the best catalytic effect on lithium oxalate decomposition, significantly reducing the decomposition overpotential of lithium oxalate, allowing it to decompose at 4.15V.

[0117] Experimental Example 9

[0118] PE separators, modified PE separators with low decomposition voltage and high lithium replenishment capacity prepared in Examples 1-2, and modified PE separators prepared in Comparative Example 1 were assembled into button cells with nickel-cobalt-manganese 811 as the positive electrode and lithium foil as the negative electrode (separator coating side facing the positive electrode). These button cells were activated at a 0.05C rate with a charge / discharge voltage range of 2.8V-4.3V. Comparison of charge / discharge curves for different button cells during activation was obtained, as shown in the figure. Figure 19 As shown in the figure. A comparison of the first-cycle lithium replenishment capacity during the activation process of different button batteries, as shown in the figure. Figure 20 As shown. Figure 19 The results show that the battery assembled in Example 1 exhibits the highest specific charge capacity during the activation process. Figure 20 It can be seen that the battery assembled in Example 1 has the highest lithium replenishment capacity during the first charge cycle in the activation process.

[0119] Experimental Example 10

[0120] The modified PE separator with low decomposition voltage and high lithium supplement capacity prepared in Example 1-Example 2 and the modified PE separator prepared in Comparative Example 1 were respectively assembled into lithium symmetric batteries. The constant current charging time was set to 1 h, the constant current charging current was 0.5 mA, the constant current discharging time was 1 h, the constant current discharging current was 0.5 mA, and the cycle number was 1000 times during the cycle performance test. The cycle stability of different lithium symmetric batteries was compared by performing cycle performance tests on the batteries, and the comparison chart is shown in Figure 21 As can be seen from Figure 21 , under the same cycle conditions, the lithium symmetric battery assembled in Example 1 has the best cycle stability.

[0121] Experimental Example 11

[0122] The modified PE separator with low decomposition voltage and high lithium supplement capacity prepared in Example 1-Example 2 and the modified PE separator prepared in Comparative Example 1 were respectively assembled into full batteries with nickel-cobalt-manganese 811 as the positive electrode and silicon-carbon material as the negative electrode. The cycle performance of the batteries was tested at 0.5C rate, and the voltage range of the charge and discharge was set to 2.8V-4.3V, and the cycle curves of different full batteries were compared, as shown in Figure 22 As can be seen from Figure 22 , the full battery assembled in Example 1 has better cycle stability and capacity retention rate.

[0123] Therefore, by using the preparation method, the composite catalyst is obtained by sintering the multi-phase precursor, and the conductive agent, the binder, lithium oxalate, etc. are uniformly grinded and coated on the PE separator, which can greatly reduce the decomposition potential of lithium oxalate, effectively solve the problem that lithium oxalate is difficult to supplement lithium in the ternary positive electrode system, improve the specific capacity of lithium oxalate released at low voltage, and achieve the effect of high lithium supplement capacity.

[0124] In the composite catalyst of the present application, the empty orbital of molybdenum carbide adsorbs and catalyzes the lone pair electrons of lithium oxalate, the carbon structure with high conductivity establishes a fast transmission channel for electron and ion migration, and the introduced cobalt element can change the electron cloud distribution and surface chemical environment of molybdenum carbide, increase the active center, reduce the energy barrier of lithium release of lithium oxalate, accelerate the reaction kinetics between lithium ions and active materials, promote the embedding and de-embedding process of lithium ions, thereby improving the lithium supplement efficiency, enabling the battery to complete the charging and discharging process more quickly, and improving the rate performance of the battery.

[0125] Meanwhile, cobalt elements can also fill the lattice defects or interstitial positions of molybdenum carbide, support and reinforce the lattice structure, enhance the stability of the crystal structure of molybdenum carbide, and further improve the overall stability of the coating. In addition, the presence of the lithium supplement coating (the coating formed after the slurry is dried) improves the mechanical strength of the PE separator, and the promotion of the reaction kinetics by the composite catalyst makes the assembled battery have excellent cycle capacity retention rate and high specific capacity, and excellent cycle stability.

Claims

1. A method for preparing a modified PE separator with low decomposition voltage and high lithium replenishment capacity, characterized in that, Includes the following steps: S1. C3N4, a molybdenum source, a cobalt source, and citric acid are mixed to obtain a precursor; the molar ratio of carbon in C3N4 to molybdenum in the molybdenum source is (10-12):(1-1.2); the molar ratio of molybdenum in the molybdenum source to cobalt in the cobalt source is 10:(1-7); the molar ratio of the total molybdenum in the molybdenum source and cobalt in the cobalt source to citric acid is (0.5-1.5):(1-2). S2. The precursor is sintered under a protective atmosphere to obtain a composite catalyst; S3. Mix the composite catalyst, conductive agent, binder, lithium oxalate and solvent to obtain a slurry; the mass ratio of the composite catalyst, conductive agent, binder and lithium oxalate is (15-25):(5-15):(2-8):(60-70); S4. The slurry is coated on the surface of the PE membrane and dried to obtain a modified PE membrane with low decomposition voltage and high lithium replenishment capacity.

2. The method for preparing the modified PE separator with low decomposition voltage and high lithium replenishment capacity according to claim 1, characterized in that, The molybdenum source S1 includes one or more of ammonium molybdate and molybdenum oxide, and the cobalt source includes one or more of cobalt nitrate, cobalt chloride, and cobalt acetate.

3. The method for preparing the modified PE separator with low decomposition voltage and high lithium replenishment capacity according to claim 1, characterized in that, In S1, the mixture is prepared in a mixed solution of ethanol and water, with a volume ratio of ethanol to water of (0.5-1.5):(0.5-1.5).

4. The method for preparing the modified PE separator with low decomposition voltage and high lithium replenishment capacity according to claim 1, characterized in that, The protective atmosphere described in S2 includes one or more of argon and hydrogen; the heating rate of the sintering is 8℃ / min-12℃ / min, the target sintering temperature is 780℃-820℃, and the holding time of the sintering is 1.5h-2.5h.

5. The method for preparing the modified PE separator with low decomposition voltage and high lithium replenishment capacity according to claim 1, characterized in that, The conductive agent in S3 includes one or more of acetylene black, Ketjen black, Kayner D5315, and Lier LA205; the binder includes polyvinylidene fluoride; and the solvent includes one or more of N-methylpyrrolidone and dimethylformamide.

6. The method for preparing the modified PE separator with low decomposition voltage and high lithium replenishment capacity according to claim 1 or 5, characterized in that, The mass fraction of solids in the slurry described in S3 is 15%-25%.

7. The method for preparing the modified PE separator with low decomposition voltage and high lithium replenishment capacity according to claim 1, characterized in that, In S4, the ratio of the coating thickness to the PE membrane thickness is (1-2):(10-20).

8. The method for preparing the modified PE separator with low decomposition voltage and high lithium replenishment capacity according to claim 1 or 7, characterized in that, In S4, drying includes a first drying and a second drying performed sequentially. The first drying temperature is 20℃-30℃, and the first drying time is 2h-4h; the second drying temperature is 50℃-70℃, and the second drying time is 11h-13h.

9. A modified PE separator with low decomposition voltage and high lithium replenishment capacity, characterized in that, The modified PE separator with low decomposition voltage and high lithium replenishment capacity is prepared according to any one of claims 1-8.

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